Desalting is typically the first operation in oil refineries. Crude oil that is processed without desalting is detrimental to the refinery assets, leading to severe corrosion problems. The desalter system removes the majority of salts in the crude oil by injecting water into the system. Because of the higher solubility in water, salts move from the crude oil to the water phase. Thus, desalter systems are typically large gravity settling tanks that provide enough residence time for both the water and the crude oil to settle. Usually density of water is higher than that of oil; hence, water settles at the bottom of the desalter system, and crude oil leaves the unit from the top. Further, the addition of an electrical grid at the top of desalter systems can promote the separation of crude oil at the top and the water to settle at the bottom. Desalting can also involve chemical treatment of the crude oil and/or the water to facilitate removal of the salt.
In ideal operation, the crude oil and water should have a very thin interface. However, in practice, during the operation, an emulsion of water in crude oil is formed as a distinct layer between the water and crude oil. This emulsion band is also called a rag layer, and can be quite dynamic in position and size. Typically, these emulsion bands can cause oil refiners to run less than optimum wash water rates and less than optimum extent of shearing to facilitate oil and water mixing through a mixing device, which reduces its efficiency for salt and sediment removal. Excessive growth of these emulsion bands can shorten the operational lifespan of the electrical grids in the desalter system, thus bringing the entire refinery operations to a halt. Accordingly, it is not only important to monitor and control the performance of the desalter system, but it is also desirous to keep the position and size of the emulsion band under control.
Performance of a desalter can be characterized based on three parameters: percentage salt removal in desalted crude oil relative to that of feed, percentage water removal in desalted crude oil relative to that of feed, and percentage oil carry over in brine or desalter water exit stream. Optimal operation of the desalter means very high values of salt and water removal and close to zero value for oil carryover in water.
Furthermore, operation of the desalter system is difficult and requires an expert with vast experience to make the right corrective decision. The crude oil blend in refineries changes frequently, and when the refineries process a new blend, the operators need to be able to judge performance of the desalter system without direct visibility of the emulsion band (rag layer), to determine effectiveness of the chemical treatment, and to initiate appropriate corrective actions during upset conditions.
Thus, there exists a strong need for a method of providing advisory controls for a desalter system, which continuously monitors performance of the desalter system, continuously monitors position of the emulsion band, controls the emulsion band and provides recommendations for maintaining optimal amount of mixing of the oil and water.